In our earlier U.S. Pat. Nos. 5,224,346 and 5,992,141, we described experiments which showed that the amount of nitrogen oxides (NO and NO2), or NOx in engine exhaust gasses can be reduced by injecting ammonia (NH3) which reacts with nitrogen oxides to produce nitrogen and water. One practical problem is how to store ammonia that is to be injected into the exhaust gas stream. Ammonia can be stored as a liquid, at a pressure of about 150 psi, with the ammonia turning into gas when removed from the container and its pressure reduced. It is often undesirable to store ammonia under considerable pressure because of the danger of an explosion and the possibility of rapid leakage of ammonia. Another way for storing ammonia is to store it as urea (H2 NCO NH2) as in a solution with water, and inject it into the exhaust gas stream. However, it can take a longer time for urea to react with nitrogen oxides and turn them into nitrogen and water, than ammonia. Since the temperature of the exhaust gas stream very rapidly drops as the exhaust gasses move through the exhaust pipe into the atmosphere, it is desirable that the reactions occur very rapidly so they can occur at the high temperatures that exist in and near the exhaust gas manifold so that a high proportion of the injected material (ammonia or urea) can react with the nitrogen oxides. A system that enabled the storage of ammonia (NH3) without requiring storage under high pressure, so that the ammonia could be directly injected into hot exhaust gasses and rapidly react with nitrogen oxides, would be of value.